1. Field of the Invention
The present invention relates to engine components of steel or superalloy comprising a coating for preventing wear when subjected to alternating friction at medium temperature, i.e. in the vicinity of 700.degree. C., and also to a process for obtaining such coatings.
In turboshaft engines, for example, particularly those intended for aircraft, several parts subjected to temperatures from 400.degree. C. to 800.degree. C. undergo alternating frictions against which they must be protected. Some examples of where such frictions occur are as follows:
the centering of the combustion chamber inlet diffuser on the preceding compressor stage;
the support of an upstream inner platform of the high pressure turbine guide on the downstream flange of the combustion chamber;
the positioning of the outer platform of the high pressure turbine guide on the front and rear flanges of the high pressure turbine casing;
the fixing flange of turbine sectors on the high pressure turbine casing;
the centering of the exhaust cone on the turbine outlet casing.
2. Summary of the Prior Art
An attempt has been made to form a coating at these various places which would compensate for wear and provide a hard wear-resistant layer. For this purpose, the components at these places were subjected to a localized electrolytic deposition of cobalt with a chromium oxide Cr.sub.2 O.sub.3 phase in the cobalt bath. It has been observed, however, that although this type of coating is effective in the first two examples mentioned above, it is not at all so in the last three cases where wear and flaking, and even breaking away, of the coating has occurred.
It has been possible to correlate these observations with the temperatures experienced at the different areas, and it has been determined that electrolytic depositions of cobalt with a dispersed phase of chromium oxide only behave satisfactorily below a temperature of 700.degree. C. on a continuous basis, beyond which, flaking occurs.